This invention relates generally to rheology test apparatus and more particularly, but not by way of limitation, to a transportable rheology test system having commonly associated laminar flow rheology and computational equipment for use at a well site to predict friction pressure loss and bottom hole treating pressure, for example, during a fracturing treatment being performed on the well.
Once an oil or gas well is drilled, it is sometimes necessary to fracture a formation to improve the flowability of the hydrocarbons trapped in the formation. To perform a fracturing job, specialized fracturing fluids suitable for the specific job are blended. The fracturing fluids are developed at the well site by blending selected chemicals into a base gel, the characteristics of which gel can be controlled based upon rheological properties as determined from real-time measurements taken through the use of a laminar flow rheology loop. Such measurements can also be used for predicting friction pressure loss and bottom hole treating pressures that are likely to occur when the blended fluid is pumped downhole during the fracturing process. Such control and prediction abilities enhance the chances of obtaining a good result from the fracturing job under way, and the information obtained therefrom is helpful in designing other blends for subsequent jobs.
One technique for obtaining such control and prediction abilities is based upon the shear rate versus shear stress relationship applicable to the particular base gel being used. This relationship can be determined by monitoring differential pressures along various lengths and diameters of the flowing base gel. Measurements of such differential pressures correlate to points on a graphical representation of the particular shear rate versus shear stress relationship. For example, a least squares fit calculation can be used with the differential pressure measurements to compute a straight line having a slope representing the n' value and having a y-axis intercept representing the k' value for the shear rate versus shear stress relationship. These values are used by a computer for correlating to turbulent flow in pipes of other diameters and for providing other suitable information on a real-time basis. The mathematical relationships of this technique are known to the art.
The foregoing technique of using differential pressure measurements to develop a shear rate versus shear stress relationship that can be correlated into useful, real-time information by which the quality of a gel can be controlled and by which predictions of downhole phenomena can be made has been implemented by a portable laminar flow rheology skid developed and used by Halliburton Services prior to October, 1984. This skid is transportable by a vehicle dedicated to that function, such as a pickup truck, but it is electrically connectible to a COMPUVAN.TM. testing vehicle for providing the differential pressure information to computers in the testing vehicle, which computers generate the correlations and predictions based on the differential pressure information. This skid, however, is not designed so that it can be carried by a testing vehicle such as the COMPUVAN.TM. testing vehicle, thereby necessitating the use of two separate vehicles to transport both the skid and the necessary computational equipment to the well site. This prior skid is also relatively difficult to disassemble and clean.
In operation, the prior skid has been used with a blending system that includes frac tanks containing the base gel, a blender tub, a suction pump for pumping the gel from the frac tanks to the blender tub, a discharge pump for pumping the blend from the tub for subsequent high pressure pumping, by other pumps, into the well. Additives can be blended into the base gel through application into the tub or along the flow path as known to the art. The prior laminar flow rheology loop has an inlet connected to receive a portion of the clean base gel tapped from the flow between the suction pump and the tub. The rheology loop has an outlet through which the tapped portion flows into the tub.
The tapped flow circulates through at least part of the loop, which comprises four test pipe sections having different diameters (e.g., nominal inner diameters of 3/8 inch, 1/2 inch, 3/4 inch and one inch). A respective differential pressure transducer and two respective pressure taps are used to measure the pressure drop over a specified length in each pipe section. To increase the range of gel viscosities that can be accommodated in this flow loop, only three test pipe sections are used by the computer program at any one time (in this embodiment, either the three larger pipe sections or the three smaller pipe sections). If a relatively thin fluid is being used, for example, then lower flow rates or larger pipes will keep the flow laminar. To check if the flow is laminar, the computer program makes a linear regression analysis of the differential pressure measurements from the test pipe sections and computes a correlation coefficient, R. The value of R should approach 1.0. A value of R much less than 1.0 would indicate non-laminar flow in one or more pipes or perhaps a plugged pressure tap.
To prevent the pressure taps, from which the pressure measurements are taken, from becoming plugged as well as to remove air from the lines connecting these taps to the differential pressure transducers associated with the rheology loop, a purge system is provided on the flow loop. This system includes a reservoir of purge fluid, a purge pump and eight on-off toggle valves. The purge pump supplies a suitable flow, and the toggle valves allow each tap to be purged individually. Such purging helps to insure that each pressure tap will respond as designed.
In this prior rheology loop, the flow rate of the base gel being circulated through the loop must also be controlled and measured. The flow rate is controlled by a 1-inch ball valve, and the flow rate is measured in one of two different ways, depending on the type of gel being circulated. If an agueous (conductive) gel is circulated through the loop, a magnetic flow meter measures the flow rate. If a non-aqueous (non-conductive) gel is circulated, a standard turbine flow meter measures the flow rate.
The prior skid also includes a pH probe and temperature probe placed in the flow stream to monitor these characteristics of the base gel.
The prior skid also includes a bypass valve so that the 3/8-inch test pipe section can be removed from the flow stream. This is used not only to select between which set of three test pipe sections is to be used, but also to bypass the smallest pipe should the outlet pressure of the blender tub become marginal whereby such bypassing decreases the back pressure that the laminar flow loop creates in the portion of the flow taken from the blender tub.
Although this prior skid functions satisfactorily, there is the need for an improved rheology test system constructed so that a rheology flow loop skid thereof can be conveniently and readily transported on a complementally constructed vehicle also capable of carrying the computation equipment needed to perform the necessary calculations. It is also desirable for such an improved loop to be constructed so that at least parts of it can be more easily disassembled and the loop more easily cleaned than in the prior skid.